Traffic information providing system and method for generating traffic information

ABSTRACT

The invention relates to a method for generating traffic information to be used in a car navigation system, comprising the steps of acquiring traffic information data including information relating to a travel time of links making up a road on a map and for acquiring map data relating to the link, the map data comprising at least information on a road type of the link and checking if a volume of the traffic information data available in is sufficient for performing a statistical estimate for the travel time. In order to enhance the traffic information generated, it is proposed to use the steps of checking if the volume of traffic information data relating to links located in a target area is sufficient and adapting a size of the target area depending on the amount of available traffic information data.

BACKGROUND OF THE INVENTION

The present invention relates to a traffic information providing systemfor creating traffic information to be used in a car navigation system.In particular, the invention relates to a traffic information providingsystem suitable for generating traffic information relating to freetraffic flow conditions. Moreover, the invention relates to a method forgenerating traffic information to be used in a car navigation system, adata storage device storing traffic information generated according tothe method of the invention and to a car navigation system comprisingsuch a data storage device.

The document US2005/0093720A1 teaches a traffic information providingsystem for creating traffic information to be used in a car navigationsystem. The traffic information providing system comprises a unit forgetting traffic information data relating to a travel time of linksmaking up a road on a map. The data getting unit moreover acquires mapdata relating to the link, wherein the map data comprises e.g.information on the road type of the link, on a length of the link and onthe coordinates of endpoints of the link. The system according toUS2005/0093720A1 teaches to delete abnormal data when there is anabnormality in the acquired traffic information and to apply astatistical processing for calculating a probability distribution oftravel times of links making up the entire route and to calculate aprobability for a specific travel time range. If no traffic informationis provided for a specific link, the travel time associated to such linkis estimated either based on a congestion degree and a congestion linkof the non-provision link or estimates the travel time associated to thelink based on traffic information of links in the vicinity of thenon-provision link. The latter estimate uses traffic informationrelating to links within a predetermined distance of e.g. 2 kilometers.

In a technique disclosed in JP-A-7-129893, the average vehicle speed ofa link where no traffic information is provided is calculated based ontraffic jam or congestion information included in external information.The complementary information for the non-provision link is calculatedbased on a calculated average vehicle speed and the travel time for thelink is also estimated based on the average vehicle speeds at specifictimes of the day or at specific days of the week (e.g. holidays, workdays, etc.). The technique disclosed in JP-A-7-129893 is intended to beapplied a for vehicle route guiding system. However, traffic informationis updated from moment to moment, and when traffic information datareceived are also included in the traffic information database used forcalculating the average, a required memory capacity and correspondinglya required processor capacity becomes enormous. In such a vehicle routeguiding system, it is difficult to estimate a travel time of anon-provision link on a real-time basis, since the memory capacity andthe processing performance are limited. In addition, it is veryinefficient to perform the same data processing procedures forindividual vehicle route guiding systems over and over again.

The document JP-A-2002-260142 discloses a technique where data aboutruntimes of a road section including the road in question are collectedand stored in a runtime memory table having runtime data base whereruntime data are collected. In order to estimate a runtime, the runtimememory table is searched for a pattern similar in runtime and a runtimememory value for the similar pattern found is used as a predictedruntime. By repeating such operations, a plurality of candidates of thepredicted runtime are found. In the technique disclosed inJP-A-2002-260142, even when a prediction error within a givenprobability is found, the error may fail to fall in a practical range.For example, the prediction error within a probability of 90% may becomelarger than a mean value of the probability distribution of traveltimes, such that the thus acquired statistical information is not of anypractical use.

In general, the estimation of travel times or links where no trafficdata is provided is performed based on either road traffic sensor dataor on the use of limiting speed data. Since the road traffic sensors aremanaged by public authorities, it is generally difficult to get suchdata. The limiting speed data are rarely provided in map data. However,map data and traffic information data often include average travel timesrelating to a specific link. Conventional traffic information providingsystems therefore often use the average travel times provided by thedata providers as an estimate for the travel time under free trafficconditions for links where no detailed traffic information is provided.However, the average travel times provided in the map data are often ofpoor quality, such that the estimates based on such average travel timesoften suffer from large estimation errors. Since the travel timeestimates for free traffic flow are often used as a basis for estimatesunder congested conditions, the estimation error will be propagated tofurther processing steps and tends to increase.

In view of the above, it is very important to provide a precise estimateof the travel times under free traffic flow conditions at e.g. midnightas a sound basis for further estimation procedures using the lattertravel time estimate or free traffic flow conditions.

SUMMARY OF THE INVENTION

One of the objects of the invention is to provide a traffic informationproviding system for creating traffic information to be used in a carnavigation system which accurately estimates the travel speeds in freetraffic flow conditions, using both real traffic data and map data.Moreover, the invention seeks to avoid the use of externally providedaverage travel speed data provided by map data providers as far aspossible.

According to one aspect of the invention, the invention starts from atraffic information providing system for creating traffic information tobe used in a car navigation system. The traffic information providingsystem comprises a data getting unit for acquiring traffic informationdata including information relating to a travel time of links making upa road on a map and for acquiring map data relating to the link. The mapdata may comprise at least information on a road type of the link andmay further comprise information on a length and on the coordinates ofthe starting point and of the endpoint of the link. The information onthe road type may include information on the number of lanes of the roadmaking up the link. Moreover, the traffic information providing systemcomprises a data volume check unit for checking if a volume of thetraffic information data available is sufficient for performing astatistical estimate for the travel time.

In particular, the invention proposes to configure the data volume checkunit in a way that the data volume check unit checks the volume oftraffic information data relating to links located in a target area andto adapt a size of the target area depending on the amount of availabletraffic information data. Due to this configuration, the size of thetarget area can always be suitably chosen, such that the volume ofavailable data relating to the links located in the chosen target areais sufficient to achieve a trustworthy prediction while the target areais still small enough to account for the regional variations in thetraffic conditions.

According to a further aspect of the invention, it is proposed that thedata volume check unit is configured to increase the size of the targetarea, if a volume of traffic information data relating to links locatedin a previously chosen target is insufficient. Due to thisconfiguration, the size of the target area can be stepwise adaptedstarting from low values, such that an excessively large target area canbe surely avoided.

If the traffic information providing system comprises a speed processingunit for determining the travel speed of a specific link by evaluatingthe traffic information data relating to links of the same or similarroad type and being located in the same target area as the specificlink, erroneous estimates due to a mixing of different road types can beavoided. The road type may be differentiated according to the number oflanes.

Moreover, it is proposed that the speed processing unit is configured toselect a predetermined percentile speed from a distribution of speedvalues of speed values extracted from the traffic information data,wherein the speed values are relating to links of the same or similarroad type and wherein the links are located in the same target area asthe specific link. Moreover, the speed processing unit may be configuredto set the value of a tentative speed of the specific links equal to thevalue of the predetermined percentile speed. If a percentile speed isselected, the influence of single abnormal data points may be weakenedas compared to approaches where the tentative speed is selected based onan average and/or variance of the distribution of speed values.

It has turned out that very viable predictions can be achieved if thepredetermined percentile speed is higher than the 60^(th) percentilespeed. Favorably, the predetermined percentile speed is between the80^(th) and 90^(th) percentile speed. Most favorably, the predeterminedpercentile speed is chosen as the 85^(th) percentile speed of all thespeed data arranged in an ascending order. In order to the percentilespeed, all the speed data are ordered in an ascending or descendingorder and the number of the available speed data points is determined.The total number of available speed data points is multiplied with afactor between 0 and 1, corresponding to the predetermined percentilevalue and the result is rounded up or down to the next integer value.The speed value according to the rank of the thus determined integervalue is selected.

Furthermore, it is proposed that the traffic information providingsystem is provided with a speed compensation unit being configured tocalculate a speed value for a specific link dependent on a tentativespeed value determined from a distribution of speed values. Thedistribution of speed values may be extracted from the trafficinformation data and/or from a data base of previously stored speedvalues. The speed compensation unit may calculate the speed value forthe specific link by applying at least one correction function to thetentative speed value. It has turned out that estimates being directlybased on the statistics and/or on the tentative value suffer fromestimation errors being, among others, due to an erroneous measurementof traffic sensors and to erroneous data in the traffic informationdata. By selecting suitable speed compensation methods, the viability ofthe speed estimates may be highly increased.

In particular, it is proposed that the correction function is amonotonically increasing function with values between a lower speedlimit and an upper speed limit. Such a correction function may filterout speed values beyond the upper speed limit and below the lower speedlimit. Excessively low speed values may be due to errors in the speedmeasurement, e.g. if a car is illegally parking within the range of aspeed sensor. Other errors in the traffic information data generatingmethods may lead to excessively high speed values, which may be cut offusing the upper speed limit.

A corruption of viable speed data between the lower speed limit and theupper speed limit may be avoided if the correction function is equal tothe identity function for values of the tentative speed between a firstthreshold and the upper speed limit. The first threshold may correspondto the lower speed limit.

A very simple and fast correction of the speed data may be achieved ifthe correction function is a piecewise linear function.

According to a further aspect of the invention, it is proposed that thespeed compensation unit is configured to calculate a characteristicvalue for a road density within an area comprising the specific link andto determine the correction function depending on the characteristicvalue. It has turned out, that the road density of an area a routepasses through strongly influences the travel time. A higher roaddensity leads to a longer travel time and vice versa. Therefore, it isfavorable to slightly augment the travel time estimates for areas withhigher road density and to decrease the travel times for areas withlower road density. If such an approach is chosen, a route searchalgorithm using the traffic information data generated by the trafficinformation providing system according to the invention will tend toavoid areas with higher route density. The finally calculated route willcircumvent such areas, which leads to a better result in the timeestimates and to an increased driving comfort for the driver. In view ofthe above, it is particularly favorable, if the correction function hasa slope that decreases when the road density decreases.

Moreover, it is proposed that the speed compensation unit is configuredto consequently apply at least a first correction function and a secondcorrection function to the tentative speed value. The differentcorrection functions may account for different sources of estimationerrors.

If the traffic information providing system comprises a speed adjustmentunit being configured to compare average speeds of different road typeswithin the same area and to adjust the average speeds if the comparisonyields to an unexpected result, contradictions in the estimated speedsmay be avoided. If e.g. the speed estimate for interstate highways islower than the speed estimates for smaller streets, the route searchalgorithm using the data would avoid interstate highways in favour ofother road types and wrong travel time estimates would be generated. Thedifferent road types may be arranged according to the predeterminedspeed rank order for a given number of lanes. The road types may includeinternational and intercity motorways, national motorways, internationaland intercity highways, national and other highways, district roads,arterial roads, basic roads and ferry routes.

If the speed adjustment unit is configured to determine the ratio of theadjusted average speed and the original average speed for each road typeand to adjust the speed values for each link of the same road type inthe same area by multiplying it with the thus determined ratio, theadjustment of the average speeds may be propagated to the speed valuesof the individual links.

According to a further aspect of the invention, the method forgenerating traffic information to be used in a car navigation system isproposed. The method comprises the steps of acquiring trafficinformation data including information relating to a travel time oflinks making up a road on a map and for acquiring map data relating tothe link. The map data comprises at least information on a road type ofthe link and may comprise further information, e.g. on the length of thelinks and on coordinates of endpoints of the link.

The method further comprises the step of checking if a volume of thetraffic information data available is sufficient for performing astatistical estimate for the travel time. According to the invention,the step of checking if the volume of the traffic information datarelating to links located in a target area is sufficient comprisesadapting a size of the target area depending on the amount of availabletraffic information data. According to the above described method, thetarget area may be chosen always in an optimal way such that it issufficiently large to perform a viable statistical analysis on the onehand and that on the other hand smaller length scale variations in thetraffic characteristics are kept as far as possible.

According to a further aspect of the invention, it is proposed toprovide a data storage device, which may be formed e.g. as a hard discdrive or as an optical disk, wherein the data storage device comprisestraffic information generated according to the above mentioned method.

Finally, according to a further aspect of the invention, it is proposedto provide a car navigation system comprising a storage device of theabove described type, wherein traffic information is generated accordingto the above method are stored.

Furthermore, it is proposed to provide a car navigation system or atruck planning system performing at least one of the steps of the methodaccording to the invention. In particular, the speed compensation couldbe performed by a speed compensation unit located of the car navigationsystem, wherein the car navigation system reads traffic information dataas output by the statistical processing step.

Further objects and advantages of the invention will become apparentfrom the following description of schematic drawings. The descriptionand the drawings illustrate a specific embodiment of the inventioncombining a multitude of features, the merits of which will beappreciated by the skilled person individually or in other suitablecombinations. The specific embodiment as described below is not intendedto limit the general idea of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the traffic information providing systemaccording to the invention.

FIG. 2 is flow chart representing a speed data estimation unit of thetraffic information providing system according to FIG. 1.

FIG. 3 is a schematic representation of a target area extension foradapting a size of the target area as performed by a data volume checkunit of the speed data estimation unit of FIGS. 1 and 2.

FIG. 4 is a graph of a first compensation function for the compensationof abnormal data.

FIG. 5 is a graph of a second compensation function representing arelationship between a road density and a coefficient for compensation.

FIG. 6 is a schematic representation illustrating a calculation of aroad density as carried out by a speed compensation unit of the speeddata estimation unit according to FIG. 1.

FIG. 7 is a flow chart representing the method for adjusting travelspeeds among different road types.

FIG. 8 is a sample of a route and predicted arrival time provided by acar navigation system according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a schematic representation of a traffic information providingsystem for creating traffic information to be used in a car navigationsystem. The traffic information providing system comprises a speed dataestimation unit 13 which processes traffic information data acquiredfrom a traffic data base 11 and map data acquired from a map data base12. The traffic information storage in the traffic data base 11 isprovided by a traffic data aggregation unit 10.

The speed data estimation unit 13 processes the information in order toobtain speed data 14. The speed data is a data structure where linksforming a map are associated with predetermined travel speeds or traveltimes. The speed data 14 estimated by the speed data estimation unit 13is compiled by a data compilation unit 15 and subsequently stored in astorage device 16 comprising route search data. The route search data istraffic information which is usable by a car navigation system, in orderto perform a route search. The car navigation system uses well-knownroute search algorithms in order to find an optimum route from astarting point to an endpoint, wherein at least one optimizationcriterion is the travel time. The travel time for a given route iscalculated by the car navigation system as a sum of the travel timesassociated to the links making up the route.

In order to obtain the traffic information data and the map data, thespeed data estimation unit 13 comprises a data getting unit 130 foracquiring the traffic information data and the map data. The trafficinformation data comprises a large number of sample travel timesassociated to the links making up the map. Moreover, the trafficinformation data may include live data such as data relating to trafficcongestions, accidents, etc.

The map data is data representing a road map, wherein the road map isdivided into level 1 and level 2 meshes, wherein each mesh includestypically more than one link and more than one road. The links withinthe meshes are stored as data structures comprising a linkidentification number, an identification of the mesh comprising thelink, coordinates of the staring points and endpoints of the links, alength of the link and an average travel time associated to the link.

The data obtained by the data getting unit 130 is processed in a seriesof steps by a data volume check unit 131, a statistical unit 132 and aspeed compensation unit 133, in order to obtain the speed data. Thespeed data includes travel speeds from every link for every time span ofa day and for different types of days (e.g. holidays, work days).

In the embodiment described herein after, the method according to theinvention is applied to estimate travel speeds and/or travel times forlinks where no sufficient traffic data is available for immediatelydetermining the travel speed or the travel times from a statisticalprocessing. In practice, real traffic data can be obtained from thetraffic data aggregation unit 10 only for major arterial roads andmotorways or highways at a sufficient extent, whereas no real trafficdata is available for the majority of smaller district roads or lessimportant road sections. The invention is mainly concerned withestimating the travel speed for the links where no real traffic data isprovided under free traffic flow conditions. The travel times forcongested conditions may be estimated based on the estimate for freetraffic flow conditions, using other known methods.

The processing of the data volume check unit 131, the speed processingunit 132 and the speed compensation unit 133 is described in more detailwith reference to FIG. 2.

FIG. 2 shows a flow chart of the processing performed in the speed dataestimation unit 13. In a first step (Step 20), the traffic data and mapdata is obtained by the data getting unit 130. In a second step (Step21), the volume of the traffic information data available in a giventarget area (FIG. 3) is checked. The volume check of the traffic data isperformed for each road type and each number of lanes individually. Thedifferent road types include national and international motorways,national and international highways, district roads, arterial roads,basic roads and ferry routes. If a travel speed estimate for a linkrepresenting a road of the same road type, e.g. an international highwaywith two lanes, is sought, the data volume check unit checks if withinthe given target area, there is sufficient traffic data for this roadtype, in the above example for international highways with two lanes. Ifon the other hand the speed data estimation unit 13 is estimating speeddata e.g. for a link corresponding to a district road, the data volumecheck unit checks whether there is sufficient travel data of districtroads within the given target area.

The data volume check unit 131 calculates the necessary number N of datapoints from a T distribution table depending on a confidence interval α,a standard deviation of speeds in an area, an average speed in an areasuch that a given target accuracy (e.g. 10%) for the speed estimate isachievable. The volume check is performed based on the traffic datavolume at midnight or, in other words, for free traffic conditions inevery area, for every road type and for every number of lanes.

If the available volume of traffic data exceeds a predeterminedthreshold calculated as described above, it is judged as “OK”. If thevolume is not sufficient, such that a statistical processing wouldresult in an accuracy below the desired target accuracy, the data volumecheck unit adapts the size of the target area depending on the amount ofavailable traffic information data. The adaption of the size of thetarget area is performed by increasing the size of the target areastepwise (FIG. 3), until the available data volume within the increasedtarget area is either sufficient or until an upper limit of the areaextension is reached. The parameter for the upper limit of the areaextension and the parameter for judging the sufficiency of data volumeare determined in advance. For typical applications in Central Europe,the upper limit for the area extension corresponds e.g. to a few tens ofkilometers. The initial target area corresponds to one level 1 mesh. Themeshes are defined depending on the geographic latitude and longitudeand essentially correspond to squares with edges of e.g. roughly 2.5kilometers. Level 2 meshes have edges with a length of e.g. roughly 10kilometers.

The data volume check unit 131 sends the result of the data volume check(OK/NOT OK) to the speed processing unit 132 (Step 22).

As shown in FIG. 3, the area extension is performed by including level 1meshes neighboring the previous target area into an increased area, suchthat the original or initial target area always remains in the center ofthe extended target area.

The speed processing unit 132 tentatively determines the speed in freetraffic flow conditions. If the volume check unit is “NOT OK”, even ifthe upper limit for the area extension is reached, the speed processingunit calculates an average speed on the area, road type and number oflanes by averaging the average speeds obtained from the map data storedin the map data base (Step 23). On the other hand, if the data volumecheck unit 131 has sent a result “OK”, the speed processing unit 132processes the traffic data statistically by estimating the travel speedfor the link where no real traffic data is available from availabletraffic data relating to the same or similar road types within the sametarget area.

The statistical processing is performed by calculating the percentilespeed determined in advance. Refer to the Step 24. The speed processingunit 132 arranges the available speed data points in an ascending orderand selects the 85^(th) percentile speed. By doing so, the influence ofisolated abnormally high speed data points is excluded.

The selected tentative speed is sent to a speed compensation unit 133which compensates the tentative speed data calculated by the speedprocessing unit 132. The speed compensation unit 133 executes threecompensation methods.

The first method is compensation of abnormal data. Refer to the Step 25of FIG. 2. The method, if the ratio of tentative speed V to upper limitspeed Vu is less than a predetermined threshold Rmax (<1.0), the speed Vis compensated as Vc1 in the way as shown in FIG. 4. Cmin is a parameterof minimum ratio of compensated speed Vc1 to the upper limit speed Vuand it should be less than Rmax. The upper limit speed Vu depends on theroad type and is determined in advance.

As shown in FIG. 4, the correction function is a monotonicallyincreasing function with values between the lower speed limit (Rmax·Vu)and the upper speed limit (Vu). The correction function is equal to theidentity function for values of the tentative speed between a firstthreshold Rmax·Vu and the upper speed limit Vu. The correction functionis a piecewise linear function.

The second method is a compensation by road density. Refer to the Step26 of FIG. 2. The speed compensation unit 133 is configured to calculatea characteristic value (a_(m), k_(m)) for a road density within theinitial target area or within the extended target area comprising thespecific link and determines a correction function depending on thecharacteristic value a_(m). The compensated speed Vc2 is calculated bycompensating the speed value V by multiplying V with the factor a_(m).Vc2=a _(m) ·V

Here, the speed V is the speed compensated by the compensation functionaccording to FIG. 4. The compensation parameter a_(m) is defined as alinear function of the road density K_(m), which is calculated as shownin FIG. 5. The function a_(m) decreases with increasing K_(m).a _(m) =b·k _(m) +c

The parameters b and c for this linear function are predetermined usinga statistical optimization method (e.g. least square method) using realtraffic data. The above function is shown in FIG. 5.

Road density K_(m) is calculated in the following way. (See FIG. 6)

(i) Target area is divided into some grids.

(ii) Label of grids in which straight line of both end-points of thelink is involved is set 1. If the straight line passes on lattice point,label of all the next grids (4 grids) of the lattice point is set 1.(Grids 53 in FIG. 6)

(iii) Label of several grids next from the grid of label 1 is set 2.(Grids 54 in FIG. 6) However, label of grid whose label has been 1 isnot changed.

(iv) Road density K_(m) is calculated in equation (3).Km=(ΣLi)/(S1+S2)  (3)L is length of link i. S1 and S2 is respectively superficial content oflabel 1 and 2.

The characteristic value K_(m) is calculated in a way schematicallyillustrated in FIG. 6. First, the target area is divided into grids.Second, each of the meshes of the grid, where a straight line connectingthe endpoints of the links passes through, is assigned to a first weightfactor (e.g. 1). If the straight line passes a lattice point, the gridmeshes neighboring the lattice point are set to the first weights. Themeshes which are weighted with the first weight factor are dashed inFIG. 6.

Subsequently, in the third step, the meshes of the grid neighboring amesh assigned with a first weight factor are assigned to a second weightfactor. In the present embodiment, the first weight factor is 1 and thesecond weight factor is 2. If the mesh has been assigned to the firstweight factor in the second step, its weight is not changed in the firststep. Finally, the road density K_(m) is calculated. In order to do so,the sum of all weight factors of all meshes within the target area iscalculated. The sum may be written as S₁+S₂, wherein S₁ is the sum ofall meshes associated with the first weight, and wherein S₂ is the sumof all meshes associated to the second weight. Moreover, the sum of thelength of all links within the target area is calculated. Thecharacteristic value K_(m) for the road density is calculated as afraction of the sum of the length of the links and the sum of the weightfactors.

$K_{m} = \frac{\sum\; L_{i}}{S_{1} + S_{2}}$wherein L_(i) is the length of the link i.

In a third method, the travel speeds are adjusted among the differentroad types. The adjustment is performed in one target area according toa predetermined speed rank order of road types. Refer to the Step 27 ofFIG. 2.

Road Type Speed Rank Road Type ID Order Motorway (International &Intercity) 1 1 Motorway (National & Others) 2 2 Highway (International &Intercity 3 3 Highway (National & Others) 4 4 Other Roads (District) 5 5Other Roads (Arterial) 6 6 Other Roads (Basic) 7 7 Ferry Route 8 8

FIG. 7 is a flow chart of the processing performed by the speedadjustment unit of the speed data estimation according to FIG. 1. In afirst step 70, the speed adjustment unit calculates the average speed Vjof a given road type of a speed rank j for each number of lanes and foreach area. For averaging, the speed compensation unit 133 may use thecompensated speed values according to the above methods or may use, inalternative embodiments, the original speeds as obtained from thetraffic data base 11.

In a next step (Step 71), a maximum speed of all calculated averagespeeds within a given area is extracted. In the Step 72, the speedcompensation unit 133 compares the maximum speed max(V_(j)) with allcalculated average speeds V_(k)′. If the maximum speed is not the speedfor roads of speed rank order 1, namely for international and intercitymotorways, in the Step 73, V_(k) and max(V_(j)) are adjusted accordingto the following equations:

$V_{k}^{\prime} = \frac{V_{k} + {\max\left( V_{j} \right)} + {\Delta\; V}}{2}$$V_{j}^{\prime} = \frac{V_{1} + {\max\left( V_{j} \right)} + {\Delta\; V}}{2}$

Essentially, the order of the average speed of the road with the highestspeed rank and of the average speed corresponding to the maximum averagespeed are exchanged in a way that the sum of the two speed values iskept constant and that a predetermined speed margin Δv (e.g. 2 km/h) ismaintained. If the maximum speed corresponds to the speed associated tothe road type or at the highest speed rank value, the latter speed isfixed and the above described procedure is repeated with the remainingspeed values, until all average speeds are fixed. Refer to Step 74.

Finally, the speed compensation unit 133 determines a ratio of theadjusted average speeds and the original average speeds for each roadtype and for each number of lanes and adjusts the speed values for eachlink of the same road type and of the same number of lanes in the sametarget area by multiplying the speed value with the ratio of theadjusted average speed and the original average speed. If e.g. theadjusted average speed of a district road is 20% higher than theoriginally calculated average speed for district roads within the giventarget area, the speed values associated to all the district roadswithin the target area are increased by 20%. The adjusted link speed ascalculated by multiplying the originally link speed resulting from theabove speed compensation methods with the calculated ratio. In otherwords, the link speed is compensated using an average compensation ofthe entire target area. Refer to Step 75.

The above described travel information providing system implements acorresponding method for generating traffic information and providesaccurate travel speed for all links for free traffic flow conditions. Ifthe travel speed is applied to a travel planning system such as a carnavigation system or a truck management system, the quality of route andthe predicted arrival time provided by the systems is highly improved.Because the accuracy of the travel speed is improved where no data isprovided, the total travel time for routes including covered links andnon-provision links is improved, compared with conventional systems.

A sample of a route and a predicted arrival time provided by the carnavigation system is shown in FIG. 8. The car navigation systemcomprises a display 80, representing a current location 81 and adestination 82, together with a route 83 calculated by the carnavigation system based on the speed data as calculated by the speeddata estimation unit 13. Moreover, the display 80 shows the current time84 and route information 85 including a predicted arrival time.

Moreover, the car navigation system uses the travel speed calculated bythe speed data estimation unit 13 for free traffic flow conditions forestimating a traffic speed for congested traffic flow conditions. Sincea precision of the estimate for free traffic flow conditions isimproved, the resulting estimation error in the congested traffic flowcondition estimate may be highly decreased.

The above traffic information providing system and the above method forgenerating traffic information enhances the quality of the results of aroute search algorithm. The invention accurately estimates travel speedson all road sections in free traffic flow conditions using real trafficdata, whereas data obtained from road traffic sensors or limiting speedmay be avoided.

The invention enables to estimate accurate travel speeds on all roadsections in free traffic flow conditions. The estimated speed for freetraffic flow conditions is available for accurate estimation incongested traffic flow as well. Car navigation systems or truck planningsystems can improve the quality of routes and the accuracy of thearrival time estimates by using the estimated speed data for routesearch.

The above features of the embodiment may be combined in any suitable waypartly or as a whole.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A method for generating traffic information to be used in a carnavigation system, comprising the steps of: acquiring trafficinformation data including information relating to a travel time oflinks making up a road on a map and for acquiring map data relating tothe link, said map data comprising at least information on a road typeof the link; checking if a volume of the traffic information dataavailable in is sufficient for performing a statistical estimate for thetravel time; checking if the volume of traffic information data relatingto links located in a target area is sufficient; and adapting a size ofthe target area depending on the amount of available traffic informationdata; and further comprising a speed processing step for determining atravel speed of a specific link by extracting and evaluating the trafficinformation data relating to links of the same or similar road type andbeing located in the same target area as the specific link; wherein insaid speed processing step, if an amount of the extracted trafficinformation data in the target area before the size is increased orafter the size is increased is sufficient, a predetermined percentilespeed is selected from a distribution of speed values of the extractedtraffic information data extracted from said traffic information dataand relating to links of the same or similar road type and being locatedin the same target area as the specific link and the value of atentative speed on the specific link is set to be equal to the value ofsaid predetermined percentile speed, if the amount of the extractedtraffic information data in the target data after the size is increasedis insufficient, an average speed of the distribution of the speedvalues relating to the specific link located in the target area is setto be the value of the tentative speed on the specific link.
 2. A methodfor generating traffic information to be used in a car navigationsystem, comprising the steps of: acquiring traffic information dataincluding information relating to a travel time of links making up aroad on a map and for acquiring map data relating to the link, said mapdata comprising at least information on a road type of the link;checking if a volume of the traffic information data available in issufficient for performing a statistical estimate for the travel time;checking if the volume of traffic information data relating to linkslocated in a target area is sufficient; and adapting a size of thetarget area depending on the amount of available traffic informationdata; and further comprising a speed compensation step, wherein atentative speed value determined from a distribution of speed valuesextracted from said traffic information data is subjected to at leastone correction function; wherein said correction function is amonotonically increasing function with values between a lower speedlimit and an upper speed limit.
 3. A method for generating trafficinformation to be used in a car navigation system, comprising the stepsof: acquiring traffic information data including information relating toa travel time of links making up a road on a map and for acquiring mapdata relating to the link, said map data comprising at least informationon a road type of the link; checking if a volume of the trafficinformation data available in is sufficient for performing a statisticalestimate for the travel time; checking if the volume of trafficinformation data relating to links located in a target area issufficient; and adapting a size of the target area depending on theamount of available traffic information data; and further comprising aspeed compensation step, wherein a tentative speed value determined froma distribution of speed values extracted from said traffic informationdata is subjected to at least one correction function; wherein in saidspeed compensation step, a characteristic value for a road densitywithin an area comprising the specific link is calculated and thecorrection function is determined depending on said characteristicvalue.
 4. A method for generating traffic information to be used in acar navigation system, comprising the steps of: acquiring trafficinformation data including information relating to a travel time oflinks making up a road on a map and for acquiring map data relating tothe link, said map data comprising at least information on a road typeof the link; checking if a volume of the traffic information dataavailable in is sufficient for performing a statistical estimate for thetravel time; checking if the volume of traffic information data relatingto links located in a target area is sufficient; and adapting a size ofthe target area depending on the amount of available traffic informationdata; and further comprising a speed compensation step, wherein atentative speed value determined from a distribution of speed valuesextracted from said traffic information data is subjected to at leastone correction function; wherein said speed compensation step comprisesthe step of subsequently applying at least a first correction functionand a second correction function to the tentative speed value.
 5. Amethod for generating traffic information to be used in a car navigationsystem, comprising the steps of: acquiring traffic information dataincluding information relating to a travel time of links making up aroad on a map and for acquiring map data relating to the link, said mapdata comprising at least information on a road type of the link;checking if a volume of the traffic information data available in issufficient for performing a statistical estimate for the travel time;checking if the volume of traffic information data relating to linkslocated in a target area is sufficient; and adapting a size of thetarget area depending on the amount of available traffic informationdata; and further comprising a speed compensation step, wherein atentative speed value determined from a distribution of speed valuesextracted from said traffic information data is subjected to at leastone correction function; and a speed adjustment step, wherein averagespeeds of different road types within the same area are compared andwherein the average speeds are adjusted if the comparison yields anunexpected result.
 6. The method according to claim 5, wherein saidspeed adjustment step comprises the step of determining a ratio of theadjusted average speed and the original average speed for each road typeand the step of adjusting the speed values for each link of the sameroad type in the same area by multiplying the speed values with theratio.
 7. A traffic information providing device that generates trafficinformation for a car navigation system, comprising: a data getting unitthat acquires traffic information data including information relating toa travel time of links making up a road on a map and map data relatingto the link, the map data comprising at least information on a road typeof the link; a data volume check unit that determines whether a volumeof the traffic information data is sufficient for performing astatistical estimate for the travel time, determines whether the volumeof traffic information data relating to links located in a target areais sufficient, and adapts a size of the target area depending on theamount of available traffic information data; and a speed compensationunit, in which a tentative speed value determined from a distribution ofspeed values extracted from the traffic information data is subjected toat least one correction function; wherein the correction function is amonotonically increasing function with values between a lower speedlimit and an upper speed limit.
 8. A traffic information providingdevice that generates traffic information for a car navigation system,comprising: a data getting unit that acquires traffic information dataincluding information relating to a travel time of links making up aroad on a map and map data relating to the link, the map data comprisingat least information on a road type of the link; a data volume checkunit that determines whether a volume of the traffic information data issufficient for performing a statistical estimate for the travel time,determines whether the volume of traffic information data relating tolinks located in a target area is sufficient, and adapts a size of thetarget area depending on the amount of available traffic informationdata; and a speed compensation unit, in which a tentative speed valuedetermined from a distribution of speed values extracted from thetraffic information data is subjected to at least one correctionfunction; wherein, in the speed compensation unit, a characteristicvalue for a road density within an area comprising the specific link iscalculated and the correction function is determined depending on thecharacteristic value.
 9. A traffic information providing device thatgenerates traffic information for a car navigation system, comprising: adata getting unit that acquires traffic information data includinginformation relating to a travel time of links making up a road on a mapand map data relating to the link, the map data comprising at leastinformation on a road type of the link; a data volume check unit thatdetermines whether a volume of the traffic information data issufficient for performing a statistical estimate for the travel time,determines whether the volume of traffic information data relating tolinks located in a target area is sufficient, and adapts a size of thetarget area depending on the amount of available traffic informationdata; and a speed compensation unit, in which a tentative speed valuedetermined from a distribution of speed values extracted from thetraffic information data is subjected to at least one correctionfunction; wherein the speed compensation unit subsequently applies atleast a first correction function and a second correction function tothe tentative speed value.
 10. A traffic information providing devicethat generates traffic information for a car navigation system,comprising: a data getting unit that acquires traffic information dataincluding information relating to a travel time of links making up aroad on a map and map data relating to the link, the map data comprisingat least information on a road type of the link; a data volume checkunit that determines whether a volume of the traffic information data issufficient for performing a statistical estimate for the travel time,determines whether the volume of traffic information data relating tolinks located in a target area is sufficient, and adapts a size of thetarget area depending on the amount of available traffic informationdata; a speed compensation unit, in which a tentative speed valuedetermined from a distribution of speed values extracted from thetraffic information data is subjected to at least one correctionfunction; and a speed processing unit, in which average speeds ofdifferent road types within the same area are compared and the averagespeeds are adjusted if the comparison yields an unexpected result. 11.The traffic information providing device of claim 10, wherein the speedprocessing unit determines a ratio of the adjusted average speed and theoriginal average speed for each road type and the step of adjusting thespeed values for each link of the same road type in the same area bymultiplying the speed values with the ratio.